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the calculations required to accomplish such tasks took hours; even worse, the robot could not cope with changes such as a rearrange- ment of the objects in its special environment, let alone deal with the infinitely more complex conditions found on a battlefield. Shakeys top-down approach could not pre-factor in every possibility, and it produced an entity far less adaptable than a human or, for that matter, a dog or cat, which knows how to avoid obstacles even in a strange environment.

In the mid-1980s, robotics researcher Rodney Brooks (then at Stanford University, now at MIT) found himself dissatisfied with this kind of limited performance and began questioning the value of the symbolic approach.Speaking ofintelligence without representation,he proposed that robots could act intelligently without using internal symbols.The mobile units he built could be called stupid, in that their programming and computing power were less rich than Shakeys, and instead of the brainbeing localized the processors were distributed throughout the robots to control their individual parts. Further, the sensors that detected how the robot interacted with the real world were closely tied to the motors that controlled its actions, so that the unit could respond rapidly to the data flowing in.

The result was that Brookss robots evolved their own behavior as they explored the world. For example, one of his early efforts, called Genghis, learned to walk.Although wheeled robots have their uses, a robot with legs manages better in rough terrain, which might be en- countered when NASA sends robotic explorers to distant planets. Insectlike, Genghis had six legs, each with its own motors, processor, and sensors that registered what the leg was doing.Additional sensors detected obstacles in the robots path. Others reacted to heat, enabling Genghis to sense the presence of warm-blooded mammals; for ex- ample, people.

Initially Genghiss six legs were uncoordinated and the robot could not walk. But as each leg tried different movements, Genghis learned from its mistakes through a form of behavior modification by positive and negative reinforcement. In 1990, Brooks described how the unit was programmed:

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